0 Copyright
Volume 7 ,Number 6,June 1988
1988
American Chemical Society
A Laser Flash Photolysis Study of Photochemical Carbonyl Substitution in M(CO)6 (M = Cr, Mo, W) with 1,lO-Phenanthroline Shigero Oishit Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Received June 18, 1987
Photoreactions of M(CO)6 (M = Cr, Mo, W)with 1,lO-phenanthroline(phen) have been investigated by laser flash photolysis. A transient spectrum due to Cr(C0)5(phen),where phen ligates in a monodentate fashion, was observed. In the cases of Mo and W, the growth curve of the final product, Mo(CO)4(phen), was found to accord with the kinetics of consecutive first-order reactions. These facts indicate that, even with a ligand as well-suited to bidentate bonding as phen, the coordination of M(C0l6to phen does not concert with the extrusion of the second carbonyl. Both these reactions, however, were found to be significantly accelerated compared with those for pyridine derivatives, suggesting a profound electronic interaction between the metal atom and the noncoordinated nitrogen in M(CO),(phen). The wavelength shift of the d-d band in Cr(CO),(phen) is also attributable to this interaction. However, at this stage, it cannot be concluded whether the interaction is coordination, that is, whether M(CO)&phen)can be considered as the 7-coordinate 20-electron complex of an associative substitution mechanism. Introduction The photochemical carbonyl substitution of group 6 metal carbonyls (M(CO),, M = Cr, Mo, W) is well established to start with decarbonylation by the absorption of light. T h e coordinatively unsaturated species so generated, M(CO)5, reacts with a ligand, L, to give M(C0)5L.1 M(CO)6 -% M(C0)5 + co M(CO)5 + L
-
M(CO)5L
side of the C2-C2, bond in L.6 The two nitrogens of 1,lO-phenanthroline (phen) are constrained always to be geometrically suitable for bidentate ligation. We have examined, by means of laser flash photolysis, whether phen brings about the extrusion of the second carbonyl concertedly with the coordination.
(1) (2)
In the case of a bidentate ligand, does the elimination of the second carbonyl take place concertedly with the initial coordination? M(CO)5 + L
-
M(CO)4L + CO
(3) According to studies by Lees et al., the elimination of the second carbonyl does not occur concertedly with the coordination.2” Indeed, in the case that a bidentate ligand can take a conformation where the direction of the second coordination point is not suitable for coordination, e.g., in the case of di-2-pyridylmethane, these authors were able to isolate M(CO)5L, in which L ligates in a monodentate fashion. ET IR studies of the derivatives of 2,2’-bipyridine also suggest a stepwise mechanism including M(C0)5L where two nitrogens of 2,2’-bipyridines are not on the same ‘Present address: Department of Chemistry, University of Tsukuba, Sakura-mura, Ibaraki 305, Japan.
2 , 4 ‘ -bpy
phen
Experimental Section Materials. Samples of M(CO)6(M = Cr, Mo, W) were purchased from Aldrich Chemical Co. and were used without further (1) Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry; Academic: New York, 1979. (2) Schadt, M. J.; Gresalfi, N. J.; Lees, A. J. Inorg. Chem. 1986, 24, 2942. (3) Schadt, M. J.; Lees, A. J. Inorg. Chem. 1986, 25, 672. (4) Marx, D. E.; Lees, A. J. Organometallics 1986, 5, 2072. (5) M a x , D. E.; Lees, A. J. Inorg. Chem. 1987, 26, 620. (6) Kazlauskas, R. J.; Wrighton, M. S. J. Am. Chem. SOC.1982, 104, 5784.
0216-1333/88/ 2307-1237$01.50/0 0 1988 American Chemical Society
Oishi
1238 Organometallics, Vol. 7, No. 6, 1988 0 10
01
-
L
0
-100
000
400 nm
ps
-
x
10-4s
I
400
500
600 nm
Figure 1. Laser flash photolysis of Cr(CO)6 (1.8 mM) and phen (23.8 mM) in benzene. (a) A typical growth trace at 400 nm. Solid circles were for the spectra in (b). (b) Time-resolved differential spectra. A, B, C, and D correspond to 0, 19.5,58.6,and 214.9 ws, respectively. purification. phen, bpy, 2,4'-bpy, and 2,9-dimethyl-l,lOphenanthroline (Aldrich)were recrystallized from ethanol-water and dried on P205in vacuo. Pyridine (Aldrich, Gold label) was distilled and dried with molecular sieves 4A. Benzene (Fisher Scientific,spectrophotometry grade) was pursed with the usual sulfuric acid treatments, distillation, and an alumina column and stored on molecular sieves 4A. Laser Flash Photolysis. A Molectron UV-400 nitrogen laser was used for excitation (331nm, 3 mJ/pulse, 8 ns of pulse width). A right-angle optical system using a 10-mm cell was employed for the excitation-analpingsetup. All measurements were carried out at ambient temperature (24 A 1 "C) and were the results of averages of at least four laser pulses. The estimated uncertainties in the rate constants obtained are A20 %. Details of the kinetic spectrophotometer and data collection system are described else where.'^^ Samples were deaerated by bubbling argon (Linde, Ultra high purity grade) for 20 min and stirred with a magnetic stirrer during the experiment. A flow system was used for measuring transient spectra.
Results and Discussion Benzene solutions of Cr(CO)6(1.18 mM) and phen (23.8 mM) were deaerated by bubbling argon and then subjected to laser flash photolysis at 337 nm. The absorbance change a t 400 nm is shown in Figure la. The fast decay immediately after the excitation pulse, which appears as a spike on the rising trace, is due to the decay of triplet phen generated by absorption of the excitation pulse.g Transient spectra (Figure Ib) are shown for times after this decay is completed. Cr(C0I5 (Ama = 620 nm) formed in perfluoromethylcyclohexanehas been reported to convert swiftly to Cr2(CO)11(Amm = 480 nm).loJ1 A similar phenomenon has been observed in n-perfluorohexane, and, in the presence of benzene, Cr(CO), reads to give Cr(CO),(B) (B = benzene) (A- = 460 nm), where benzene coordinates (7)Das, P.K.; Encinas, M. V.; Small, Jr., R. D.; Scaiano, J. C. J . Am. Chem. SOC. 1979,101,6965. (8)Nagarajan, V.; Fessenden, R. W. J. Phys. Chem. 1985,89,2330. (9)Teply, J.; Mehnert, R.; Brede, 0.;Fojtic, A. Radiochem. Radioanal. Lett. 1982,53,141. (10)Kelly, J. M.; Long, C.; Bonneau, R. J. Phys. Chem. 1983,87,3344. (11) Breckenridge, W.H.; Stewart, G. M. J. Am. Chem. SOC. 1986,108, 364.
I
200 M''
100
I/[LI
Figure 2. (a) Pseudo-first-order kinetics plots of decay (0) at 520 nm and growth ( 0 )at 410 nm for Cr(CO)6 (1.23 mM) and phen (17.8 mM) in benzene. (b) Plots of l / k p s vs 1/[L] based On eq 10. Table I. Apparent Coordination Rate Constants of Pyridines and phen to Cr(CO), in Benzene L PY
k,,
M-'s-l
A,
(Cr(C0)5L),nm
A(measd),nm
bpy
1.2 x 105 2.1 x 104 405 410 400 (G)" 400 ( G ) 500 (D" 500 (D)
'Abbreviations: G, growth; D,
2,4'-bpy
phen
1.5 x 105 410 400 (G) 500 (D)
8.2 x 105
440 410 (G) 520 (D)
decay.
as a q2-ligand.12 Free Cr(CO)5was not observed in benzene solution with nanosecond time resolution; spectrum A in Figure l b corresponds to Cr(C0)5(B).13 In this concentration of phen, Cr(CO),(B) changed into the second species within -200 ns,14 showing an isosbestic point at 470 nm. As shown in Figure 2a, both the growth at 410 nm and the decay at 520 nm obeyed pseudo-first-order reaction kinetics, and the reciprocals of the rate constants (l/k& were approximately linear in 1/[phen], as described later. The second transient ,A( = 440 nm) does not have the characteristic MLCT bands of Cr(CO)4(phen)(519 nm), and no band is evident up to 800 nm. The photoreaction initiated by the laser pulse cannot be different from that initiated by continuous light, since the benzene solutions irradiated by a few tens of pulses of nitrogen laser showed the spectra of Cr(CO),(phen). The spectral change can be most plausibly explained by the idea that transient spectrum D is attributed to Cr(CO)5(phen) in which phen ligates in a monodentate fashion. The phen ligand coordinates through its nitrogen. The possibility of Cr(C0)5(5,6-~2-phenanthroline) can be eliminated by the fact (12)Oishi, S.J. Organomet. Chem. 1987,335, 207. (13)Spectra observed just after the laser pulse in benzene solutions of M!CO)e (A, = 460,395,and 400 nm for M = Cr, Mo, and W, respectively)are not due to M(CO)5(solvent impurity). The impurity must be present more than -0.125 M in concentration for completing the reaction of free M(C0)6 within the duration of the laser pulse (8ns >> l/(k[impurity])),assuming even the diffusion-controlledrate for k (-lo9 M-l s'l). As written in the Experimental Section, benzene was purified specially to eliminate nucleophiles. (14) The first transient (460nm) produced by the photolysis of Cr(C0 )alone ~ in benzene did not show any change in its spectrum at this time scale. The second transient cannot be attributable to solvent impurities.
Organometallics, Vol. 7,No. 6, 1988 1239
Photochemical Carbonyl Substitution in M ( C 0 ) 6 that appearance of the second transient was very slow when 2,9-dimethyl-l,lO-phenanthroline replaces phen as the ligand. In order to follow the second carbonyl extrusion from Cr(CO),(phen), we monitored absorption a t 520 nm following the spectrum D, since the MLCT band of Cr(CO),(phen) appears at 519 nm., However, only very slight growth was observed up to 3.5 ms. The rate constant of decarbonylation (k-co) in Cr(CO)5(phen) is roughly estimated to be less than The absorption maximum of Cr(CO),(phen) occurs at 440 nm, a longer wavelength than those of typical d-d bands of Cr(CO),(py) (py = pyridine, 405 nm); and Cr(CO),(N1'-~-2,4'-bpy)(2,4'-bpy = 2,4'-bipyridine, 410 nm) (Table I). Distortions from octahedral coordination caused by a purely steric crowding around Cr cannot explain this shift because the absorption maximum of Cr(CO),(bpy) (bpy = bipyridine) observed in the photolysis of Cr(CO)6with bpy occurs at 410 nm. The bpy ligand will promote comparable crowding around Cr, but the noncoordinated nitrogen does not face Cr.6J"19 We believe that the shift in Cr(CO),(phen) is attributable to an electronic interaction between the noncoordinated nitrogen and Cr, which may give rise to distortion from a typical octahedral coordination.20 Considering that the dissociative process predominates in the thermal carbonyl substitution of M(CO)621J2and that Cr(CO),(B) will dissociate B more easily than M(CO)6 dissociates CO, reactions following the photochemical generation of free Cr(CO), will be postulated as follows.23* kb
M(CO),
+ B ck - M(CO),(B)
(4)
M(CO),
+L
(5)
M(CO),(L)
As the concentration of free Cr(CO), is very low in benzene, the steady-state treatment can be applied to this system and gives eq 6.
If kB[B] >> kL[L] holds, eq 6 can be simplified to eq 7
where
Therefore, the second-order rate constant (k,,,), obtained (15)Nakamoto. K.J. Phvs. Chem. 1960. 64. 1420. (16)Cumper, C. W.N.; Ginman, R. F. A:; Vogel, A. I. J. Chem. SOC. 1962, 1188. (17)Cureton, P.H.; LeFevere, C. G.; LeFevere, R. J. W. J. Chem. SOC. 1963, 1736. (18) Spotswood, T. Mcl.; Tanger, C. I. Aust. J. Chem. 1965,20, 1227. (19)Castellano, S.;Gunther, H.; Ebersole, S. J.Phys. Chem. 1965.69, 4166. (20)If we assume an octahedral wedge type or a ntagonal bipyramid type distorti~n,a@*~~ the n orbital of the noncoorrnated nitrogen will interact with one of the t2, orbitals to raise its energy level. And the distortion from a regular octahedral lowers both or one of the egorbitals. Therefore, the d-d band of de complex (t2, e,) is expected to shift to a longer wavelength. (21)Graham, J. R.;Angelici, R. J. Inorg. Chem. 1967, 6, 2083. (22)Covey, W.D.;Brown, T. L. Inorg. Chem. 1973, 12, 2820. (23)Bonneau, R.;Kelly, J. M. J. Am. Chem. SOC.1980, 102, 1220. (24)Lees, A. J.; Adamson, A. W. Inorg. Chem. 1981,20, 4381.
-
600
Figure 3. Time-resolved differential spectra for MO(CO)~ (1.24 mM) and phen (11.1mM) in benzene.
from the plots of pseudo-first-order rate constants (k,) vs [L] on the basis of eq 7, consists of a term involving the equilibrium of solvent coordination ( k - ~ / ( k ~ [ B land ) ) the real rate constant for complexation of L (kL).10p23,24 In this study, It,, was obtained from the plots of l/kps vs 1/[L] based on
=-.- 1 kapp
-B
500 X /nm
400
1 [LI
+ -k1-
~
In the case of the reaction of Cr(CO), with phen, the value k, using eq 7 or 10 was found to be 8.4 X lo5 or 8.2 X 1OPM-' s-l, respectively, indicating that the approximation k,[B] >> kL[L] is appropriate. Values of k,,, for py, bpy, and 2,4'-bpy are shown in Table I. Since the steric environment of N1 in 2,4'-bpy is similar to that of N in bpy, k, for N1 in 2,4'-bpy can be estimated from the value of bpy. The significantly larger value of k, for 2,4'-bpy can thus be attributed to complexation a t An electronwithdrawing 2-pyridyl substituent in 2,4'-bpy has no effect on k, ,as can be seen from the comparison with k, for py. $Re value kapEfor phen should be a t most twice kapp for py because there are twice the possible coordination sites. Instead, k,,(phen) is 6.8 times larger than kapP.(py). This indicates a rate enhancement due to the additional nitrogen. In the case of bpy, k, is far smaller than that of py because of steric hindrance caused by the protons a t C3, and C3.3 This contrasts with the fact that the lone-pair electrons of the adjacent nitrogen in phen assist the initial coordination, Figure 3 shows the spectral change of benzene solutions of MO(CO)~ (1.24 mM) and phen (11.1mM) after the excitation pulse. Mo(CO),(B) (Ama = 395 nm), present just after the pulse, converts to the final product, Mo(CO)((phen), in -1 ms.13 A significant growth at 490 nm is due to the characteristic MLCT band of M ~ ( C O ) ~ ( p h e nIf) . ~ the reaction proceeds in accord with eq 3, since phen is present in excess, the growth should obey pseudo-firstorder kinetics similar to the growth of Cr(CO),(phen). Comparing Figure 4 with Figure 1, it is obvious that the growth of Mo(CO)4(phen)shows an induction period and is not well described as a pseudo-first-order reaction. Again, the extrusion of the second carbonyl does not occur concertedly with the initial coordination. The induction period in the appearance of the MLCT band reminds us
, hit.
Oishi
1240 Organometallics, Vol. 7, No. 6, 1988 l a
32
6 3
134 m M
1
05
10
I5
t/ms
Figure 4. Growth traces ( 0 )at 490 nm for M O ( C O )(1.24 ~ mM) and phen (3.2,6.3, and 13.4 m M ) in benzene and simulated curves for k = 2700,5400, and 14000 s-l, respectively, and k x o = 5600 s-l. xbsorptions are normalized;a is 0.469,0.477, and 0.472 for 3.2,6.3, and 13.4 mM of phen, respectively. Inset: Plots of l/k, vs 1/[L]based on e q 10. Table 11. Apparent Coordination Rate Constants and Rate Constants of the Second Carbonyl Dissociation k,,, M-l c1 M(C0)6 L = phen L = py ratiod kxo,s-l Cr 8.2 x 106 1.2 x 105 6.8
